1. Technical Field
The invention relates to an image reading apparatus such as an image scanner and an image reading lens system mounted on the image reading apparatus.
2. Description of the Related Art
Recently, image reading apparatuses for scanning an original such as a document or an image and reading the original as digital image data has come into widespread use. Known image reading apparatuses include a copier, a facsimile, an image scanner, and a multi-function device having these functions and the like. These image reading apparatuses irradiate light onto an original placed on a given position to form an image of the original on a solid-state imaging device such as CCD by using light reflected from the original through a lens system, thereby obtaining image data for accurately reproducing the original.
In order to downsize the image reading apparatus, a lens (hereinafter, may be referred to as an “image reading lens system”) mounted on the image reading apparatus is provided near an original. Thus, a wide angle of view is required for the lens system. Furthermore, in order to obtain image data for accurately reproducing an original, the image reading lens system is required to have a high resolving power and have small aberrations with an excellent balance.
Recently, accompanied with a decrease in size of the image reading apparatuses, a reduction in cost of the image reading apparatuses, and an increase in resolution of the image reading apparatuses, the image reading lens system is further required a decrease in its size, a reduction in its cost, and an increase in its resolving power.
As an example of an image reading lens system for achieving a wide angle of view and a high resolving power, JP 2005-17506 A (corresponding to US 2004/0264008 A) and Japanese Patent No.3740370 (corresponding US 2002/0135890 A) have proposed six-lens configurations.
The image reading lens system described in JP 2005-17506 A and Japanese Patent No.3740370 exhibit excellent optical performance in an angular range where a half angle of view is about 25 degrees or less, but a field curvature increases in an angle of view greater than that range. In this point of view, when the image reading lens system described in JP 2005-17506 A and Japanese Patent No.3740370 is used, it is necessary to secure a spacing between the image reading lens system and an original so that the entire size of the original is included in the range of the half angle of view of 25 degrees or less.
Accordingly, a size of the image reading apparatus is restricted by the spacing between the original and the image reading lens system. Thus, there arises a problem that it is difficult to further decrease a size thereof. Conversely, when priority is given to a decrease in size of the image reading apparatus, the spacing between the original and the image reading lens system should be small. However, in this case, there arises another problem that a size of an original which can be read with a fine image quality is restricted to be relatively small.
Furthermore, since a high-intensity light source for illuminating an original is disposed close to the image reading lens system, the image reading lens system tends to be affected by heat generated in the high-intensity source. Accordingly, when the image reading lens system includes a lens made of resin, a focus tends to be changed due to thermal expansion. On the other hand, when the image reading lens system includes a glass mold lens having small thermal expansion, a focus is scarcely changed due to thermal expansion. However, the glass mold lens should be used even in a lens having an aspheric surface in order to correct aberrations. Thus, costs of the image reading lens system and the image reading apparatus increase.
Moreover, JP 2005-17506 A and Japanese Patent No.3740370 describe that image reading lens systems have an F number of 4.99 to 5.04. However, in order to increase a speed of reading an original while keeping a high resolving power, it is required for an image reading lens system to be faster than those lens systems and to suppress aberrations.